Our understanding of emphysema, the airspace destruction and enlargement in COPD, greatly outweighs our knowledge regarding the airway component, particularly acute exacerbations of COPD. Two seminal observations in the 1960s led to the elastase:antielastase hypothesis which remains the central theme in the pathogenesis of emphysema. First, was the experimental finding that instillation of elastases led to emphysema in animal models, and second was the clinical finding that patients with deficiency in alpha-1- antitrypsin (A1 AT) were at increased risk for emphysema. As was done for emphysema 40 years ago, in this proposal we will apply 21st century versions of animal models and human genetics in an attempt to launch our understanding of acute exacerbations which greatly lags behind our understanding of emphysema. Our overall hypothesis is that the risk and outcome of acute exacerbations in COPD are determined by the environmental etiology combined with genetic susceptibility. Thus, in this proposal, we will generate murine models of acute exacerbations in COPD combining cigarette smoking with viral and bacterial infection, and we will apply gene targeted mice to dissect pathogenetic pathways of acute exacerbations with an emphasis on inflammatory cells and proteinase effects on fighting infection, airway remodeling and subsequent emphysema. We will also test our hypothesis that polymorphisms in candidate genes for COPD susceptibility and innate and adaptive immunity genes will influence the frequency and severity of COPD exacerbations. We will develop a population of patients with moderate to severe COPD (FEV1 <50% predicted) and will classify the patients as either non-frequent (0) or frequent (2 or more per year) "exacerbators" based upon their clinical course during the three years before the study. Single nucleotide polymorphisms (SNPs) in twenty candidate genes will be studied for genetic association with COPD exacerbations in 400 frequent exacerbators and 400 non-frequent exacerbators. Candidate genes for COPD susceptibility will be selected from COPD linkage studies and previous case-control genetic association studies;candidate genes for innate and adaptive immunity will be selected based on the animal model studies in Aim 1.